Valves responsive to the manifold vacuum and engine torque of internal combustion engines in vehicles are known, for instance see U.S. Pats. No. 2,895,561 and 2,926,892 as representative of the prior art. Generally such devices are principally concerned with improved combustion efficiency and lower emission pollutant levels in the exhaust gases of an internal combustion engine rather than the noise created by its operation.
Conventionally, for instance, in designing heavy duty, truck-type diesel, or compression ignition type internal combustion engines, the cooling system, including the fan and radiator are sized to accommodate the amount of heat dissipated with the engine operating at maximum load at at maximum expected ambient temperature. During such operation the fan is operated at maximum speed which is an undesired source of noise and of concern to those charged with the protection of the environment. Further, since the engine may be operated at full load at abnormally high coolant temperatures, it is subject to damage under such operating conditions.